title: Understanding scanning tunneling microscopy contrast mechanisms on metal oxides: a case study.
abstract: A comprehensive analysis of contrast formation mechanisms in scanning tunneling microscopy (STM) experiments on a metal oxide surface is presented with the oxygen-induced (2√2×√2)R45° missing row reconstruction of the Cu(100) surface as a model system. Density functional theory and electronic transport calculations were combined to simulate the STM imaging behavior of pure and oxygen-contaminated metal tips with structurally and chemically different apexes while systematically varying bias voltage and tip-sample distance. The resulting multiparameter database of computed images was used to conduct an extensive comparison with experimental data. Excellent agreement was attained for a large number of cases, suggesting that the assumed model tips reproduce most of the commonly encountered contrast-determining effects. Specifically, we find that depending on the bias voltage polarity, copper-terminated tips allow selective imaging of two structurally distinct surface Cu sites, while oxygen-terminated tips show complex contrasts with pronounced asymmetry and tip-sample distance dependence. Considering the structural and chemical stability of the tips reveals that the copper-terminated apexes tend to react with surface oxygen at small tip-sample distances. In contrast, oxygen-terminated tips are considerably more stable, allowing exclusive surface oxygen imaging at small tip-sample distances. Our results provide a conclusive understanding of fundamental STM imaging mechanisms, thereby providing guidelines for experimentalists to achieve chemically selective imaging by properly selecting imaging parameters.
authors.affiliation: Physikalisches Institut, Westfälische Wilhelms-Universität Münster , Wilhelm-Klemm-Strasse 10, 48149 Münster, Germany.
authors.forename: Harry
authors.initials: H
authors.lastname: Mönig
authors.forename: Milica
authors.initials: M
authors.lastname: Todorović
authors.forename: Mehmet Z
authors.initials: MZ
authors.lastname: Baykara
authors.forename: Todd C
authors.initials: TC
authors.lastname: Schwendemann
authors.forename: Lucía
authors.initials: L
authors.lastname: Rodrigo
authors.forename: Eric I
authors.initials: EI
authors.lastname: Altman
authors.forename: Rubén
authors.initials: R
authors.lastname: Pérez
authors.forename: Udo D
authors.initials: UD
authors.lastname: Schwarz